The present invention relates to a high strength thin steel sheet (substrate for galvanizing) suitable for such uses as an automobile body and a high strength galvannealed steel sheet made from the high strength thin steel sheet, as well as manufacturing methods of the high strength thin steel sheet, the high strength hot-dip galvanized steel sheet and the high strength galvannealed steel sheet.
From the point of view of achieving a high safety, a smaller weight, a lower fuel/cost ratio, and hence cleaner earth environments, there are increasing applications of high strength steel sheets and high strength hot-dip galvanized steel sheets excellent in corrosion resistance as steel sheets for automobiles.
In order to manufacture high strength hot-dip galvanized steel sheets among others, it is necessary to previously manufacture a material sheet having a good galvanizability, and giving desired strength and workability after passing through a hot-dip galvanizing bath, and after application of a galvannealing treatment.
In order to increase strength of a steel sheet, in general, it is the common practice to add solid solution hardening elements such as P, Mn and Si and precipitation hardening elements such as Ti, Nb and V.
When a steel sheet containing these elements added as described above is treated on a continuous hot-dip galvanizing line (CGL), the steel sheet is subjected to annealing at a temperature of over the Ac1 transformation point, and further, a low cooling rate makes it difficult to obtain a high tensile strength: achievement of a high tensile strength requires addition of alloy elements in large quantities, and this leads to a higher cost.
Addition of alloy elements in large quantities is known to cause serious deterioration of galvanizing property. The quantities of added alloy elements are limited also from the point of view of galvanizability.
Because of the contradictory actions of alloy elements in the substrate steel sheet on strength and galvanizability, it has been very difficult to manufacture a high strength hot-dip galvanized steel sheet excellent in galvanizability on a continuous hot-dip galvanizing line.
In the case of high strength steel sheet, it has further been difficult to manufacture a hot-dip galvanized steel sheet excellent in workability, because of low properties relating to workability such as elongation.
As a high strength steel sheet having a high workability, on the other hand, there has conventionally been proposed a composite (containing residual austenite) mainly comprising martensite with ferrite as the base metal.
This composite structure steel sheet is non-aging at room temperature, has a low yield ratio [:{yield strength (YS)}/{tensile strength (TS)}], and is excellent in workability and hardenability after working.
A known manufacturing method of a composite structure steel sheet is to heat a steel sheet at a temperature within the (xcex1+xcex3) region, and then rapid cool the steel sheet by water cooling or gas cooling. It is also known that a higher cooling rate leads to the necessity of a smaller number of necessary alloy elements and a smaller amount of addition.
However, when a conventional composite structure steel sheet is subjected to hot-dip galvanizing at a temperature of about 500xc2x0 C., or further, to a heating-galvannealing treatment, hard martensite, a targeted secondary phase, does not occur, in addition to the primary phase ferrite, but there are generated soft cementite, pearlite and bainite. This results in a decrease in tensile strength and appearance of an upper yield point, leading to an increase in yield point, or further, an yield elongation.
Temper softening tends to be easily caused according as the quantities of added alloy elements become smaller. Large quantities of these alloy elements causes, on the other hand, a decrease in hot-dip galvanizing property.
After all, hard martensite is not generated during the galvanizing step even in the composite structure steel sheet, but soft cementite, pearlite and bainite are produced. It has therefore been difficult to achieve compatibility between workability brought about by the primary phase ferrite and a high strength based on the secondary phase martensite, and a it satisfactory galvanizability in the conventional art.
In a galvanized steel sheet, on the other hand, the galvanized steel sheet is required to be excellent in coating adhesion so as to eliminate the necessity to prevent peeling of the galvanizing layer upon press working and maintain a die.
In order to increase strength of a steel sheet, in general, it is the common practice to add solid solution hardening elements (easily oxidizable elements) such as Mn as described above. These elements however become oxides during reduction-annealing before galvanizing, are concentrated on the steel sheet surface, and reduce wettability by the molten zinc resulting in production of non-galvanized defects on the steel sheet surface in which the galvanizing layer hardly adheres to the steel sheet surface.
The cause is as follows. A recrystallization annealing atmosphere is a reducing atmosphere for Fe, which does not allow production of Fe oxides, but is an oxidizing atmosphere for easily oxidized elements such as Mn. These elements are concentrated on the steel sheet surface, form an oxide film, and thus reduce the contact area between the molten zinc and the steel sheet.
As a manufacturing method of a high strength hot-dip galvanized steel sheet, a method of regulating the cooling rate during annealing upon galvanizing is disclosed in Japanese Unexamined Patent Publication No. 55-50455. The disclosed method contains no description about a method for improving galvanizability. Particularly, when the Mn content in the material steel sheet is over 1%, it is difficult to prevent non-galvanized defects, and the method teaches nothing about a method for improving coating adhesion.
Under the current actual circumstances, therefore, the high strength steel sheet excellent in workability attraction as a high strength material for automobile lacks actual means to be applied as a surface-treated steel sheet excellent also in coating adhesion, though not excellent in workability, in the form of a hot-dip galvanized steel sheet.
Japanese Examined Patent Publication No. 7-9055 discloses a method of applying galvanizing to a steel sheet pickled after annealing as a method for improving the galvannealing rate of a P-added steel. This method has however an object to improve the galvannealing rate, not to prevent non-galvanized defects.
The above-mentioned method teaches nothing about the dew point, the hydrogen concentration and temperature of atmosphere gas upon annealing applied immediately prior to galvanizing. Non-galvanized defects are considered to occur more frequently for certain combinations of the kind of steel and the annealing atmosphere.
Japanese Unexamined Patent Publication No. 7-268584 discloses a method of conducting secondary annealing at a temperature determined in response to the P content in steel. This is however based on a technical idea that the temperature region for preventing brittleness of a steel sheet is dependent upon the P content in steel, not a disclosure of a temperature for improving galvanizability.
The present invention has an object to solve the aforementioned problems involved in the conventional art, and to provide a high strength thin steel sheet serving as a substrate for galvanizing which is excellent in workability and strength even after hot-dip galvanizing or further a galvannealing treatment, and gives an excellent galvanizability as well as an excellent corrosion resistance, a galvannealed steel sheet, made of this high strength thin steel sheet excellent in workability, coating adhesion and corrosion resistance, and manufacturing methods thereof.
More specifically, an object of the present invention is to provide a high strength thin steel sheet excellent in workability which satisfies conditions including a yield ratio of up to 70% and a TSxc3x97El value of at least 16,000 MPa. %, and permits prevention of occurrence of non-galvanized defects, a high strength galvannealed steel sheet made of the above high strength thin steel sheet, excellent in workability, coating adhesion and corrosion resistance, as well as manufacturing methods of such high strength thin steel sheet, high-strength hot-dip galvanized steel sheet and high strength galvannealed steel sheet.
As a result of extensive studies carried out to solve these problems, the present inventors obtained the following findings (1) to (4):
(1) Dispersion of Band Structures in Steel Sheet
A thin steel sheet in which a high workability and a high tensile strength are simultaneously achieved, with a satisfactory galvanizability, is available, from the point of view of improving mechanical properties, by using a steel sheet having a prescribed chemical composition and heating the steel sheet to a temperature of at least a prescribed level to cause dispersion of a band structure particularly, comprising a secondary phase (comprising mainly cementite, pearlite and bainite and only partially martensite and residual austenite) to a prescribed extent in the steel sheet.
(2) Two-stage Heating-pickling
A high strength hot-dip galvanized steel sheet, which permits prevention of non-galvanized defects, excellent in workability, coating adhesion and corrosion resistance is obtained, from the point of view of improving galvanizability, by using a steel sheet having a prescribed chemical composition, heating the steel sheet to a temperature of at least a prescribed level in an annealing furnace, then after cooling, removing a concentrated layer of steel constituents on the steel sheet surface, then annealing again the steel sheet at a prescribed heating-reduction temperature in a prescribed reducing atmosphere on a continuous hot-dip galvanizing line, and then, subjecting the steel sheet to hot-dip galvanizing.
In other words, an important point for ensuring a high galvanizability in the method of reduction-annealing a once annealed steel sheet is the atmosphere used upon reduction-annealing.
An oxide film poor in wettability with the molten zinc impairs galvanizability of the steel sheet immediately after annealing unless the atmosphere sufficiently reduces P-based pickling residues produced on the steel sheet surface upon pickling the once annealed steel sheet. In the manufacturing method of a high strength hot-dip galvanized steel sheet of the present invention, the once annealed steel sheet is annealed again at a prescribed heating-reduction temperature in a prescribed reducing atmosphere, and the subjected to hot-dip galvanizing.
(3) One-stage Heating
As a result of further studies, the present inventors obtained the following findings. Satisfactory galvanizability, workability and coating adhesion can be achieved through one-stage heating by subjecting the steel sheet to hot-dip galvanizing after heating the steel sheet at an appropriate heating temperature in an appropriate atmosphere gas.
(4) Galvannealing Treatment
A high strength galvannealed steel sheet excellent both in coating adhesion after galvannealing and corrosion resistance is available by galvannealing the hot-dip galvanized steel sheet obtained in any of (1) to (3) above preferably under conditions satisfying a prescribed galvannealing temperature.
The following aspects of the invention and preferred embodiments of these aspects of the invention (1) to (39) were completed on the basis of the aforementioned findings (1) to (4).
(1) A high strength thin steel sheet excellent in workability and galvanizability, having a composition comprising: C: from 0.01 to 0.20 wt. %, Si: up to 1.0 wt. %, Mn: from 1.0 to 3.0 wt. %, P: up to 0.10 wt. %, S: up to 0.05 wt. %, Al: up to 0.10 wt. %, N: up to 0.010 wt. %, Cr: up to 1.0 wt. %, Mo: from 0.001 to 1.00 wt. %, and the balance Fe and incidental impurities, wherein a band structure comprising a secondary phase has a thickness satisfying the relation Tb/TSxe2x89xa60.005 (where, Tb: average thickness of the band structure in the thickness direction of steel sheet; T: steel sheet thickness).
(2) A high strength thin steel sheet excellent in workability and galvanizability according to (1) above, wherein the high strength thin steel sheet further contains one or more selected from the group consisting of from 0.001 to 1.0 wt. % Nb, from 0.001 to 1.0 wt. % Ti, and from 0.001 to 1.0 wt. % V.
(3) A manufacturing method of a high strength thin steel sheet excellent in workability and galvanizability, wherein the thickness of the band structure comprising a secondary phase is adjusted within a range of Tb/Txe2x89xa60.005 (where, Tb: average thickness of the band structure in the thickness direction of steel sheet, and T: steel sheet thickness) by hot-rolling a slab having a composition comprising: C: from 0.01 to 0.20 wt. %, Si: up to 1.0 wt. %, Mn: from 1.0 to 3.0 wt. %, P: up to 0.10 wt. %, S: up to 0.05 wt. %, Al: up to 0.10 wt. %, N: up to 0.010 wt. %, Cr: up to 1.0 wt. %, Mo: from 0.001 to 1.00 wt. %, and the balance Fe and incidental impurities, coiling the hot-rolled steel sheet at a temperature of up to 750xc2x0 C., and then, after heating the steel sheet to a temperature of at least 750xc2x0 C., cooling the same.
(4) A manufacturing method of a high strength thin steel sheet excellent in workability and galvanizability according to (3) above, wherein the thickness of the band structure comprising a secondary phase is adjusted within a range of Tb/Txe2x89xa60.005 (where, Tb: average thickness of the band structure in the thickness direction of steel sheet, and T: steel sheet thickness) by coiling the hot-rolled steel sheet at a temperature of up to 750xc2x0 C., then cold-rolling the steel sheet, and then, after heating to a temperature of at least 750xc2x0 C., cooling the same.
(5) A manufacturing method of a high strength thin steel sheet excellent in workability and galvanizability according to (3) or (4) above, comprising the step of, after heating the steel sheet to a temperature of at least 750xc2x0 C., applying hot-dip galvanizing in the middle of cooling, or, after application of hot-dip galvanizing subjecting the steel sheet to a heating-galvannealing treatment.
(6) A manufacturing method of a high strength thin steel sheet excellent in workability and galvanizability according to (3) or (4) above, comprising the steps of adjusting the thickness of the band structure comprising a secondary phase within a range of Tb/Txe2x89xa60.005 (where, Tb: average thickness of the band structure in the thickness direction of steel sheet, and T: steel sheet thickness), then after heating the steel sheet to a temperature of at least 750xc2x0 C. and cooling the same, further heating the same to a temperature within a range of from 700 to 850xc2x0 C., and in the middle of subsequent cooling, subjecting the steel sheet to hot-dip galvanizing, or further to a galvannealing treatment after hot-dip galvanizing.
(7) A manufacturing method of a high strength thin steel sheet excellent in workability and galvanizability according to (5) or (6) above, wherein the coating weight of a hot-dip galvanizing layer, as represented by the coating weight per side of the steel sheet is within a range of from 20 to 120 g/m2.
(8) A manufacturing method of a high strength thin steel sheet excellent in workability and galvanizability according to any one of (5) to (7) above, wherein the coating ;weight of a galvannealed steel sheet after prescribed galvannealing heating treatment, as represented by the coating weight per side of the steel sheet is within a range of from 20 to 120 g/m2.
(9) A manufacturing method of a high strength thin steel sheet excellent in workability and galvanizability according to any one of (3) to (8) above, wherein the slab further contains one or more selected from the group consisting of up to 1.0 wt. % Nb, up to 1.0 wt. % Ti and up to 1.0 wt. %/V
(10) A manufacturing method of a high strength thin steel sheet excellent in workability and galvanizability according to any one of (3) to (8) above, wherein the slab further contains one or more selected from the group consisting of from 0.001 to 1.0 wt. % Nb, from 0.001 to 1.0 wt. % Ti and from 0.001 to 1.0 wt. % V.
(11) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to (3) above, comprising the steps of, after coiling the steel sheet at a temperature of up to 750xc2x0 C., pickling the same, heating the steel sheet to a temperature of at least 750xc2x0 C., or preferably, within a range of from 750xc2x0 C. to 1,000xc2x0 C., or more preferably, from 800xc2x0 C. to 1,000xc2x0 C. in an annealing furnace, removing the concentrated layer of steel constituents on the steel sheet surface by pickling the same after cooling, then, conducting heating-reduction under reducing conditions of P-based oxides remaining as pickling residues on the steel sheet surface, and subjecting the steel sheet to hot-dip galvanizing.
(12) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to (3) above, comprising the steps of, after coiling the steel sheet at a temperature of up to 750xc2x0 C., pickling the same, then, after cold-rolling the same, heating the steel sheet to a temperature of at least 750xc2x0 C., or preferably, within a range of from 750xc2x0 C. to 1,000xc2x0 C., or more preferably, from 800xc2x0 C. to 1,000xc2x0 C. in an annealing furnace, cooling the same, removing the concentrated layer of steel constituents on the steel sheet surface by pickling the same, then, conducting heating-reduction under reducing conditions of P-based oxides remaining as pickling residues on the steel sheet surface, and subjecting the steel sheet to hot-dip galvanizing.
(13) A manufacturing method of high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to (3) above, comprising the steps of, after coiling the steel sheet at a temperature of up 750xc2x0 C., pickling the same, then heating the steel sheet to a temperature of at least 750xc2x0 C., or preferably, within a range of from 750xc2x0 C. to 1,000xc2x0 C., or more preferably, from 800xc2x0 C. to 1,000xc2x0 C. in annealing furnace, cooling the same, removing the concentrated layer of steel constituents on the steel sheet surface through pickling, then after heating-reducing steel sheet under conditions including a dew point of an atmosphere gas within a range xe2x88x9250xc2x0 C. to 0xc2x0 C. and a hydrogen concentration of the atmosphere gas within a range of from 1 to 100 vol. %, subjecting the steel sheet to hot-dip galvanizing.
(14) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to (3) above, comprising the steps of, after coiling the steel sheet at a temperature of up to 750xc2x0 C., pickling the same, then cold-rolling the steel sheet, heating the same to a temperature of at least 750xc2x0 C., or preferably, within a range of from 750xc2x0 C. to 1,000xc2x0 C., or more preferably, from 800xc2x0 C. to 1,000xc2x0 C. in an annealing furnace, then after cooling the same, removing the concentrated layer of steel constituents on the steel sheet surface through pickling, heating-reducing the steel sheet under conditions including a dew point of an atmosphere gas within a range of from xe2x88x9250xc2x0 C. to 0xc2x0 C. and hydrogen concentration in the atmosphere gas within a range of from 1 to 100 vol. %, and then, subjecting the steel sheet to hot-dip galvanizing.
(15) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to (3) above, comprising the steps of, after coiling the steel sheet at a temperature of up to 750xc2x0 C., pickling the same, then heating the steel sheet to a temperature of at least 750xc2x0 C., or preferably, within a range of from 750xc2x0 C. to 1,000xc2x0 C., or more preferably, from 800xc2x0 C. to 1,000xc2x0 C. in an annealing furnace, then after cooling the same, removing the concentrated layer of steel constituents on the steel sheet surface through pickling, then heating-reducing the steel sheet under conditions in which the heating-reduction temperature: t1 (xc2x0 C.) satisfies the following equation (1) relative to the P content in steel: P (wt. %), and then subjecting the steel sheet to hot-dip galvanizing:
0.95xe2x89xa6{[P(wt. %)+(2/3)]xc3x971100}/{t1(xc2x0 C.)}xe2x89xa61.1xe2x80x83xe2x80x83(1)
(16) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to (3) above, comprising the steps of, after coiling the steel sheet at a temperature of up to 750xc2x0 C., pickling the same, then cold-rolling the steel sheet, heating the same to a temperature of at least 750xc2x0 C., or preferably, within a range of from 750xc2x0 C., to 1,000xc2x0 C. or more preferably, from 800xc2x0 C. to 1,000xc2x0 C. in an annealing furnace, then after cooling the same, removing the concentrated layer of steel constituents on the steel sheet surface through pickling, then heating-reducing the steel sheet under conditions in which the heating-reduction temperature: t1 (xc2x0 C.) satisfies the following equation (1) relative to the P content in steel: P (wt. %), and then subjecting the steel sheet to hot-dip galvanizing:
0.95xe2x89xa6{[P(wt. %)+(2/3)]xc3x971100}/{t1(xc2x0 C.)}xe2x89xa61.1xe2x80x83xe2x80x83(1)
(17) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to (3) above, comprising the steps of, after coiling the steel sheet at a temperature of up to 750xc2x0 C., pickling the same, then heating the steel sheet to a temperature of at least 750xc2x0 C., or preferably, within a range of from 750xc2x0 C. to 1,000xc2x0 C., or more preferably, from 800xc2x0 C. to 1,000xc2x0 C. in a annealing furnace, then after cooling the same, removing the concentrated layer of steel constituents on the steel sheet surface through pickling, then heating-reducing the steel sheet under conditions in which a dew point of the atmosphere gas within a range of from xe2x88x9250xc2x0 C. to 0xc2x0 C., a hydrogen concentration in the atmosphere gas within a range of from 1 to 100 vol. % and the heating-reduction temperature:t1 (xc2x0 C.) satisfying the following equation (1) relative to the P content in steel: P (wt. %), and subjecting the steel sheet to hot-dip galvanizing:
0.9xe2x89xa6{[P(wt. %)+(2/3)]xc3x971100}/{t1(xc2x0 C.)}xe2x89xa61.1xe2x80x83xe2x80x83(1)
(18) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to (3) above, comprising the steps of, after coiling the steel sheet at a temperature of up to 750xc2x0 C., pickling the same, then cold-rolling the steel sheet, heating the same to a temperature of at least 750xc2x0 C., or preferably, within a range of from 750xc2x0 C. to 1,000xc2x0 C., or more preferably, from 800xc2x0 C. to 1,000xc2x0 C. in an annealing furnace, then after cooling the same, removing the concentrated layer of steel constituents on the steel sheet surface through pickling, then heating-reducing the steel sheet under conditions in which a dew point of the atmosphere gas within a range of from xe2x88x925xc2x0 C. to 0xc2x0 C., a hydrogen concentration in the atmosphere gas within a range of from 1 to 100 vol. % and the heating-reduction temperature: t1 (xc2x0 C.) satisfying the following equation (1) relative to the P content in steel: P (wt. %), and subjecting the steel sheet to hot-dip galvanizing:
0.9xe2x89xa6{[P(wt. %)+(2/3)]xc3x971100}/{t1(xc2x0 C.)}xe2x89xa61.1xe2x80x83xe2x80x83(1)
(19) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to any one of (11) to (18) above, comprising the steps of heating the steel sheet at a temperature of at least 750xc2x0 C., preferably within a range of from 750xc2x0 C. to 1,000xc2x0 C., or more preferably, from 800xc2x0 C. to 1,000xc2x0 C., then after cooling the same, applying thereto a pickling method comprising the step of pickling the steel sheet in a pickling liquid having a pHxe2x89xa61, and a liquid temperature with a range of from 40 to 90xc2x0 C. for a period within a range of from 1 to 20 seconds.
(20) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to any one of (11) to (19) above, comprising the step of heating the steel sheet to a temperature of at least 750xc2x0 C., or preferably within a range of from 750xc2x0 C. to 1,000xc2x0 C., or more preferably, from 800xc2x0 C. to 1,000xc2x0 C. in an annealing furnace, wherein the pickling liquid after cooling is a hydro chloric acid solution having an HCl concentration within a range of from 1 to 10 wt. %.
(21) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to (3) above, comprising the steps of, after coiling the steel sheet at a temperature of up to 750xc2x0 C., pickling the same, then heating the same at a heating temperature: T within a range of from 750xc2x0 C. to 1,000 C. and satisfying the following equation (2) in an atmosphere gas having a dew point: t of an atmosphere gas satisfying the following equation (3) and a hydrogen concentration within a range of from 1 to 100 vol. %, and then subjecting the steel sheet to hot-dip galvanizing:
0.85xe2x89xa6{[P(wt. %)+(2/3)]xc3x971150}/{T(xc2x0 C.)}xe2x89xa61.15xe2x80x83xe2x80x83(2)
0.35xe2x89xa6{[P(wt. %)+(2/3)]xc3x97(xe2x88x9230)}/{t(xc2x0 C.)}xe2x89xa61.8xe2x80x83xe2x80x83(3)
(22) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to (3) above, comprising the steps of, after coiling the steel sheet at a temperature of up to 750xc2x0 C., pickling the same, then cold-rolling the same, then heating the same at a heating temperature: T within a range of from 750xc2x0 C. to 1,000xc2x0 C. and satisfying equation (2) in an atmosphere gas having a dew point: t of an atmosphere gas satisfying the following equation (3) and a hydrogen concentration within a range of from 1 to 100 vol. %, and then subjecting the steel sheet to hot-dip galvanizing:
0.85xe2x89xa6{[P(wt. %)+(2/3)]xc3x971150}/{T(xc2x0 C.)}xe2x89xa61.15xe2x80x83xe2x80x83(2)
0.35xe2x89xa6{[P(wt. %)+(2/3)]xc3x97(xe2x88x9230)}/{t(xc2x0 C.)}xe2x89xa61.8xe2x80x83xe2x80x83(3)
(23) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to any one of (11) to (22) above, wherein the slab further contains one or more selected from the group consisting of up to 1.0 wt. % Nb, up to 1.0 wt. % Ti and up to 1.0 wt. % V.
(24) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to any one of (11) to (22) above, wherein the slab further contains one or more selected from the group consisting of from 0.001 to 1.0 wt. % Nb, from 0.001 to 1.0 wt. % Ti, and from 0.001 to 1.0 wt. % V.
(25) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to any one of (11) to (24) above, wherein the coating weight of the high strength hot-dip galvanized steel sheet, as represented by the coating weight per side of the steel sheet, of from 20 to 120 g/m2.
(26) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to any one of (13), (14), (17), (18), (21) and (22) above, wherein, when the hydrogen concentration of the atmosphere gas is within a range of from 1 vol. % to under 100 vol. %, the remaining gas is an inert gas.
(27) A manufacturing method of a high strength hot-dip galvanized steel sheet excellent in workability and coating adhesion according to (26) above, wherein the inert gas is nitrogen gas.
(28) A manufacturing method of a high strength galvannealed steel sheet excellent in workability and coating adhesion, comprising the step of subjecting the hot-dip galvanized steel sheet obtained by the manufacturing method of a high strength hot-dip galvanized steel sheet according to any one of (11) to (27) above further to a galvannealing treatment.
(29) A manufacturing method of a high strength galvannealed steel sheet excellent in workability and coating adhesion, comprising the steps of subjecting the hot-dip galvanized steel sheet according to any one of (11) to (27) above further to a galvannealing treatment, wherein the temperature: t2 (xc2x0 C.) in the galvannealing treatment satisfies the following equation (4) relative to the P content in steel: P (wt. %) and the Al content: Al (wt. %) in the bath upon the hot-dip galvanizing:
0.95xe2x89xa6[7xc3x97{100xc3x97[P(wt. %)+(2/3)]+10xc3x97Al(wt. %)}]/[t2(xc2x0 C.)]xe2x89xa61.05xe2x80x83xe2x80x83(4)
(30) A manufacturing method of a high strength galvannealed steel sheet excellent in workability and coating adhesion according to (28) or (29) above, wherein the slab further contains one or more selected from the group consisting of up to 1.0 wt. % Nb, up to 1.0 wt. % Ti and up to 1.0 wt. % V.
(31) A manufacturing method of a high strength galvannealed steel sheet excellent in workability and coating adhesion according to (28) or (29) above, wherein the slab further contains one or more selected from the group consisting of from 0.001 to 1.0 wt. % Nb, from 0.001 to 1.0 wt. % Ti and from 0.001 to 1.0 wt. % V.
(32) A manufacturing method of a high strength galvannealed steel sheet excellent in workability and coating adhesion according to any one of (28) to (31) above, wherein the coating weight of the galvannealing layer of the high strength galvannealed steel sheet is within a range of from 20 to 120 g/m2 as represented by the coating weight per side of the steel sheet.
(33) A high strength galvannealed steel sheet excellent in workability, coating adhesion and corrosion resistance, obtained by hot-dip galvanizing a steel sheet containing up to 1.00 wt. % Mo and then subjecting the steel sheet to galvannealing, wherein, in the galvannealing layer, the Fe content is within a range of from 8 to 11 wt. %, and the Mo content is within a range of from 0.002 to 0.11 wt. %.
(34) A high strength galvannealed steel sheet excellent in workability, coating adhesion and corrosion resistance, obtained by hot-dip galvanizing a steel sheet containing up to 1.00 wt. % Mo and from 0.010 to 0.2 wt. % C and then subjecting the steel sheet to galvannealing, wherein, in the galvannealing layer, the Fe content is within a range of from 8 to 11 wt. %, and the Mo content is within a range of from 0.002 to 0.11 wt. %.
(35) A high strength galvannealed steel sheet excellent in workability, coating adhesion and corrosion resistance according to (33) or (34) above, wherein the steel sheet containing up to 1.00 wt. % Mo contains Mo in an amount within a range of from 0.01 to 1.00 wt. %, ore preferably, from 0.05 to 1.00 wt. %.
(36) A high strength galvannealed steel sheet excellent in workability, coating adhesion and corrosion resistance according to any one of (33) to (35) above, wherein the substrate steel sheet serving as the steel sheet is a steel sheet comprising a chemical composition further containing up to 1.0 wt. % Si, from 1.0 to 3.0 wt. % Mn, up to 0.10 wt. % P, up to 0.05 wt. % S, up to 0.10 wt. % Al, up to 0.010 wt. % N, up to 1.0 wt. % Cr and the balance Fe and incidental impurities.
(37) A high strength galvannealed steel sheet excellent in workability, coating adhesion and corrosion resistance according to any one of (33) to (36), wherein the substrate steel sheet serving as the steel sheet further contains one or more selected from the group consisting of up to 1.0 wt. % Nb, up to 1.0 wt. % Ti and up to 1.0 wt. % V
(38) A high strength galvannealed steel sheet excellent in workability, coating adhesion and corrosion resistance according to any one of (33) to (36) above, wherein the substrate steel sheet serving as the steel sheet further contains one or more selected from the group consisting of from 0.001 to 1.0 wt. % Nb, from 0.001 to 1.0 wt. % Ti and from 0.001 to 1.0 wt. % V.
(39) A high strength galvannealed steel sheet excellent workability, coating adhesion and corrosion resistance according to any one of (33) to (38) above, wherein the coating weight of the galvannealing layer of the high strength galvannealed steel sheet is within a range of from 20 to 120 g/m2 as represented by a coating weight per side of the steel.